1. Field of the Invention
The present invention relates to a fuel cell (hereinafter referred to as xe2x80x9cFCxe2x80x9d) system which may be provided on an electric vehicle comprising an FC and an electric energy buffer such as, for example, a rechargeable battery (hereinafter referred to as xe2x80x9cbatteryxe2x80x9d) for back up the shortage of the demand power when the transient output of the FC transient outputs and which prevents the discharge for the utilization ratio of the FC.
2. Description of Related Art
The prior art in this field is disclosed in Japanese Patent Publication No. 8-31328.
FIG. 2 is a block diagram showing the conventional FC system disclosed in the aforesaid documents.
The FC system comprises an FC 1 for generating an output current S1 by using supplied reformed gas as a fuel corresponding to the quantity of the fuel. The FC 1 includes a fuel processor (hereinafter referred to as xe2x80x9cFPxe2x80x9d) 1a. The FP 1a inputs a command value S11a of the mass of the reformed gas and supplies the reformed gas corresponding to the command value S11a to the FC 1 and further output a limit current value S1a of the FC output current S1. The FC output current S1 is detected by an FC current sensor 2. The FC current sensor 2 detects the output current S1 and output the detected FC current value S2. The FC 1 is connected to an FC current controller 3 constituted with a DC/DC converter at the output thereof. The FC current controller 3 input the FC output current S1, control the value of the FC output current S1 based on a given FC current control signal 31 and output an output current S3. The FC current controller 3 is connected to an electric energy buffer (for example, a battery) 4 for charging a part of the FC output current S3 as a charge current S3a and output a discharge current S4 at the output thereof. The battery 4 comprises a battery temperature sensor 4a for detecting the temperature of the battery 4 and output a detected battery temperature S4a. The charge current S3a and the discharge current S4 are detected by a battery current sensor 5. The battery current sensor 5 detect the charge current S3a or the discharge current S4 and output a detected current value S5. The FC current controller 3 is connected to an output voltage sensor 6 for detecting the output voltage of the battery 4 and output a detected output voltage value S6.
The temperature sensor 4a, the battery current sensor 5 and the output voltage sensor 6 are connected to a battery controller 7. The battery controller 7 inputs the detected battery temperature S4a, the detected current value S5, and the detected voltage value S6 and then calculates the state of charge (hereinafter called SOC) which shows the ratio of the remaining charge capacity to the rated capacity of the battery 4 and also calculates a battery power S7b which shows the output power of the battery 4.
The output current S3 and the discharge current S4 are detected by a load current sensor 8. The load current sensor 8 detects the output current S3 and the discharge current S4 and outputs a detected load current S8. Further, the FC current controller 3 is connected to a load drive unit 9. The load drive unit 9 inputs the output current S3 and the discharge current S4 and supply a load current corresponding to a given load control signal S10a to a load L. The load drive unit 9 is connected to a load controller 10. The load controller 10 input an input signal ac which show a demand value of the load current S9 and the detected load current value S8 and output a demand power signal S10b which show the demand load current S9 and a load control signal S10a. 
The FP 1a, the FC current sensor 2, the FC current controller 3, the battery controller 7 and the load controller 10 are connected to a controller 11. The controller 11 input the limit current value S1a, the detected current value S2, the SOC S7a, the battery power S7b and the demand signal S10b and output a command value S11a and a current controlling signal S11b. 
Next, the operation of the FC system of FIG. 2 will be explained.
The command value S11a of the mass of the reformed gas is transferred to the FP 1a from the controller 11, and then the reformed gas having the mass corresponding to the command value S11a is applied to the FC 1 from the FP 1a. The FC 1 output the FC output current S1 corresponding to the mass of fuel. The FC output current S1 is detected by the FC current sensor 2 and then the FC current sensor 2 output the detected FC current S2. Further, the FP 1a output the limit current value S1a of the FC output current S1. The current controller 3 controls the value of the FC output current S1 based on the current control signal S11b and output the output FC current S3. The part of the output current S3 is supplied to the battery 4 as the charge current S3a and the discharge current S4 is outputted from the battery 4. The battery temperature sensor 4a detect the temperature of the battery 4 and output the detected battery temperature S4a. The battery current sensor 5 detect the charge current S3a and the discharge current S4 and output the detected current value S5. The output voltage sensor 6 detect the voltage of the battery 4 and output the detected output voltage S6.
The battery controller 7 input the detected battery temperature S4a, the detected current value S5 and the detected output voltage S6 and output the SOC S7a and the power S7b of the battery. Then, the load current sensor 8 detect the output current S3 and the discharge current S4 and output the detected load current value S8. The load drive unit 9 input the output current S3 and the discharge current S4 and supply the load current S9 corresponding to the load control signal S10a to the load L. The load controller 10 input the input signal ac which show the demand value of the load current S9 and the detected load current S8 and output the demand power signal S10b and the load control signal S10a. The controller 11 input the limit current value S1a, the detected FC current S2, the SOC S7a, the battery power S7b and the demand power signal S10b and output the command value S11a and the current control signal S11b. The controller 11 supply the stable power to the load L even if the response of the FC 1 is delayed because of the large variation of the load L. Further, the controller 11 prevents the battery 4 from over discharge and over charge by correcting the generating power of the fuel corresponding to the SOC S7a of the battery.
However, the prior art battery system of FIG. 2 has the following problems.
FIG. 3 is a graph showing the characteristics of the output current S3 and the discharge current S4 of FIG. 2. The vertical axis shows the voltage and the horizontal axis shows the current.
In the FC system of FIG. 2, as shown in FIG. 3, in the region C having the output current S3 of approximately 140 A or below, the voltage of the output current S3 is larger than the voltage of the battery 4 at no load (approximately 325 V), which means that the battery 4 is normally charged from the FC 1. In the region D having the output current of approximately 140 A or above, the voltage of the output current S3 is smaller than the voltage of the battery 4 at no load, which means that the battery 4 is not charged from the FC 1.
However, the controller 11 controls the FC 1 by correcting the generating power mass of the FC 1 corresponding to the SOC S7a of the battery 4, so that the FC output current S1 is supplied from the FC 1 corresponding to the SOC S7a and the load L. Accordingly, the output current S1 contain the charge current for the battery 4 but the charge current is not used for the charge of the battery 4, which lower the utilization ratio of the FC 1. Furthermore, because the battery 4 is not charged, the SOC S7a is not increased and the mass of the fuel to the FC 1 is increased. Because of this, a fuel control system for the FC 1 comprising an off gas combustor, an evaporator and a reformer which are not shown in FIG. 2 is overheated and overrun, which extremely lower the utilization ratio of the fuel.
In order to solve the above-described problems, the FC system of the present invention comprises an FC for generating a first output current by using supplied reformed gas as a fuel corresponding to the quantity of the fuel, a fuel processor for receiving a command value of the mass of the reformed gas, supplying the reformed gas corresponding to the command value to the FC, and transmitting a limit current value of the first output current, a first current sensor for detecting a value of the first output current, a current controller constituted by a DC/DC converter for receiving the first output current, controlling the value of the first output current based on a given current control signal, and transmitting a second output current, a rechargeable battery for charging a part of the second output current as a charge current and outputting a discharge current, a battery temperature sensor for detecting the battery temperature and output a detected battery temperature, a second current sensor for detecting the charge current or the discharge current to output a second detected current value, an output voltage sensor for detecting the voltage of the battery and outputting a detected output voltage, a battery controller for receiving the detected battery temperature, the second detected current value and the detected output voltage value, calculating the SOC showing the ratio of the remaining capacity to the rated capacity, and calculating a battery power indicating the power output from the battery, a third current sensor for detecting the second output current and the discharge current to output a third detected current value, a load drive unit for receiving the second output current and the discharge current and supplying a load current to a load corresponding to a given load control signal, a load controller for receiving an input signal indicating the demand value of the load current and the third detected current value and transmitting a demand power signal indicating the demand value of the load current and the load control signal, and a controller for receiving the limit current value, the first detected current value, the SOC, the battery power and the demand load signal and transmitting the command value and the current control signal.
The controller comprises a battery demand power table for receiving the SOC and transmitting the demand power S31-1 of the input and output power of the battery corresponding to the SOC, a first subtracter for subtracting the battery power from the demand power and outputting a first subtraction result, a first PI controller (here P is proportional; I is integration) for receiving and PI-controlling the first subtraction result and transmitting a first control result, a limiter for receiving the SOC, the demand power signal and the first control result and outputting a charge power for the battery by limiting the first control result in the range of the battery power corresponding to the SOC and the demand power signal, a second subtracter for subtracting the charge power from the demand power signal and transmitting a second subtraction result, a demand power calculation unit for receiving the second subtraction result, dividing the second subtraction result by a predetermined efficiency of the current controller to calculate a demand power, a power/supply fuel converter for converting the demand generating power to a command value of the mass of the reformed gas, a power/current converter for receiving the demand power, converting the demand power into the demand current of the FC and transmitting the result, a comparing and select unit for comparing the demand current of the FC with the limit current and selecting the small one to output as a demand value of the second output current, a third subtracter for subtracting the first detected current value from the required value of the second output current and transmitting the third subtraction result, and a second PI controller for receiving and PI-controlling the third subtraction result, generating a second control result and transmitting it to the current controller as a current control signal.
According to the cell system of the present invention, when the charge power limited by the limiter is subtracted from the demand power signal, the second subtraction result is a value that the charge power to the battery is removed from the demand power signal, so that the charge power is never supplied from the FC to the battery which is charged to the maximum ratio. Accordingly, the utilization ratio of the FC can be prevented from decreasing while the battery is prevented from overcharge. Further, the FC is prevented from overrunning.
Further, when the charge power limited by the limiter is subtracted from the demand power signal, in the region of the demand power signal of a predetermined value or above, the second subtraction result is the same as the demand power signal. In the region of the demand power signal of the predetermined value or below, the second subtraction result is the value that the demand power signal is added to the charge power to the battery. Accordingly, the battery which is discharged to the minimum ratio can be prevented from over discharge.